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Saturday, February 20, 2010

Another big part of my PhD research concerns volcanic clays. Volcanoes are a really interesting example of multiple natural cycles operating at once - not only do they create new rock, they also break it down. This could be pretty quickly - blasting lava into ash in an eruption - or slowly, through hydrothermal processes. When you get a combination of hot rocks and water, you (eventually) get alteration minerals, and some of those include clays.

This is a major concern in terms of volcano stability. Clays are weaker than the rocks that they form from, and an informal term that volcanologists use for hydrothermally altered rock is "rotten rock". In some cases, the reduction in stability is a function of the clays themselves, which are layered minerals and thus likely to shear along particular planes; in other cases, the reduced stability is because of what clays do, which is block or absorb water. Clays are made up of layers of tetrahedral and octahedral structures, and depending on how those layers are arranged (and how many times the tetrahedra repeat in a layer), a clay can either act as a barrier to water, or absorb it and expand (these are known as "shrink-swell clays"). If a clay blocks water, it could create a slippery plane within a volcanic edifice, which would make it easier for the rock above it to fail and collapse. Kaolinite clays are an example of this. If a clay absorbs water and shrinks or swells, this constant movement could also destabilize the material above it (smectites and like montmorillonite are good examples here.)
But the dangers don't stop there. Clays, in a collapse of a volcanic edifice, can increase the runout of the debris or mudflow that may result; this occurred in Nicaragua in 1998 at the Castia volcano, when a hurricane caused part of an old lava dome complex to collapse, and the smectite clays that had formed in the domes from hydrothermal processes helped form a massive lahar that traveled more than 10 km. It's also a concern at a number of Cascade volcanoes, including Mount Rainier; even if the volcanoes haven't erupted in a long time, there are still active hydrothermal systems within their flanks, altering the volcanic rocks there to weaker materials. Multiple studies have attempted to map these alteration zones and determine where a collapse might occur based on the location and extent of particular clay minerals.

5 comments:

Nice summary to extrusive minerals and their alteration products. I teach a soil mechanics class - another aspect of these shrink/swell clays is that they can impart unwanted lateral forces on retaining walls and foundations. And that susceptibility-to-shearing thing: no good for slope stability, especially when subject to loads from buildings.

Although volcanic soils form one of my favorite soil taxa: "andisols."

If it weren't for volcanoes (via plate tectonics) the only minerals we'd ever really see would be clays, transition/other metal oxides, and quartz (or other forms of silicon dioxide).

Volcanic clays can be hazardous to science as well. A few years back, Petrobras drilled an exploration hole through the entire Brazilian Cryogenean sedimentary sequence. Geochronological cross-corellation of the neoptoterozoic glaciations is a big deal for understanding the Earth's surface environment at the time of multicellular radiation. And the drillers intersected several volcanic layers nicely bracketing the diamictites. Naturally, the Brazilian scientists were keen to use this to pin down the ages of the glaciations. So they ran the zircons from these ~650 Ma sediments. And what was the dominant zircon age population in each volcanic layer?

2 million years.

The drillers had used Andean bentonite as their drilling mud, introducing quaternary zircons into the entire sequence and contaminating all of their cuttings. The results were unpublishable.

Out of curiosity, why did you separate shrinking and swelling clays into smectite and montmorillonite? Isn't montmorillonite a smectite clay? Just checking if I am missing something about clay classification...